3. Is there a ban on corporate donations to political parties?

§ 25 (2) Nr. 3 Parteiengesetz (PartG)
Von der Befugnis der Parteien, Spenden anzunehmen, ausgeschlossen sind:
Spenden von außerhalb des Geltungsbereiches dieses Gesetzes, es sei denn, dass a) diese Spenden aus dem Vermögen eines Deutschen im Sinne des Grundgesetzes, eines Bürgers der Europäischen Union oder eines Wirtschaftsunternehmens, dessen Anteile sich zu mehr als 50 % im Eigentum von Deutschen im Sinne des Grundgesetzes oder eines Bürgers der Europäischen Union befinden oder dessen Hauptsitz in einem Mitgliedstaat der Europäischen Union ist, unmittelbar einer Partei zufließen,

Section 25 (2) No. 3 Political Parties Act 
The following shall be excluded from the right of political parties to accept donations:
Donations from sources outside the territorial scope of this Act, unless:
a) these donations accrue directly to a political party from the assets of a German as defined by the Basic Law, a citizen of the European Union, or a business enterprise of whose shares more than 50 percent are owned by Germans as defined by the Basic Law or by a citizen of the European Union, or whose registered office is located in a Member State of the European Union;

Available at: https://www.bundeswahlleiterin.de/dam/jcr/1aedeb82-9067-4321-acce-880ba22ddc28/parteiengesetz.pdf

The Political Parties Act § 25 prohibits corporate donations to political parties, except in narrowly defined cases of Section 25 (2) No. 3. "business enterprise of whose shares more than 50 per cent are owned by Germans as defined by the Basic Law or by a citizen of the European Union, or whose registered office is located in a Member State of the European Union.